In preliminary studies of the Rhzobium-legume symbiosia, we have promulgated a novel hypothesis: that Rhizobium sp. ORS571 conducts synergistic nicotinate oxidation and N2 fixation. We propose to study ORS571 nicotinate oxidation by the Vector-insertion mutanesis/cloning technique that we have developed. We also propose to study the biochemistry and physiology of nicotinate oxidation reactions. We propose to integrate the two approaches by using nicotinate oxidation Vector-insertion mutants obtained from the genetic studies as experimental subjects for the biochemical and physiological studies. We shall access: (a) the nicotinate catabolism genes and, (b) the genes integrating nicotinate catabolism and N2 fixation. We shall construct genetic and physical maps ORS571 nicotinate catabolism genes. We shall attempt to compare and contrast synergistic nicotinate oxidation and N2 fixation in free-living rhizobia and symbiotic "bacteroids". We also hope to assess how nicotinate-catabolizing, N2-fixing ORS571 cells actively assimilate ammonium, and whether ORS571 nicotinate oxidation genes are integrated with the regulation of general N-metabolism. In contrast to the free-living process, we hope to understand how fixed-N is charted during symbiosis in Sesbania rostrata and thus how the host plant controls a potential pathogen.